Application and Fault Analysis of Natural Gas Turbine Flowmeter

November 18, 2024

Application and Fault Analysis of Natural Gas Turbine Flowmeter

With the completion of the long-distance, large-caliber, high-pressure gas pipelines such as the West-East Gas Pipeline and the Sichuan-East Gas Transmission and the gradual formation of the gas transmission and distribution network, the pipeline operators are scientific and reliable in the measurement of gas trade. Impartiality raises high demands. Relying on the rapid development of electronic technology, Internet and computer technology, the gas metering method is also developing towards automation, intelligence and remoteness. At present, gas turbine flowmeters have realized real-time, online and remote control, and gradually change to turbine flowmeters. The development of smart metering systems consisting of transmitters, SCADA systems, and chromatographic analyzers.
Turbine flowmeters are widely used in industrial trade measurement because of their high sensitivity, good repeatability, wide range ratio and compact structure. In terms of design standards, Europe PrEN12261 is a comprehensive and targeted standard specification for gas turbine flowmeters. It has specific requirements for gas turbine flowmeters in terms of metering performance, material requirements, output, marking, documentation, etc.; other design standards also includes AGANo7, Standards organization ISO9951: 1993, legal metrology organization OIMLR137-1, Japan JISZ8765-1980 and so on. Domestically, GB/T18940-2003 is mainly used for the design of turbine flowmeters.
First, the measurement principle of the turbine flowmeter
　　 The turbine flow meter is a speed flow meter. The measuring body is a turbine flow sensor. It is mainly composed of an impeller and a magnetoelectric induction coil. The impeller axis is the same as the center of the pipeline and can be freely rotated. The blades are at an angle to the gas flow direction . Under the impact of the fluid, the turbine rotates along the axial direction of the pipe, and its rotation rate varies with the flow rate. The higher the flow rate, the higher the kinetic energy and the higher the rotational speed of the turbine. The speed of the turbine is converted into electric pulses of the corresponding frequency by the magnetoelectric induction converter, and is integrated and displayed by the display instrument.
Turbine flowmeters can display cumulative flow and instantaneous flow respectively through different calculation methods. When the turbine blade cuts the magnetic flux generated by the magnetic steel in the casing , it will cause a change in the magnetic flux of the sensing coil. The flux cycle change signal is amplified and shaped by the preamplifier to generate a pulse signal proportional to the flow velocity, and the pulse signal is subjected to a flow product. After the circuit is converted, the cumulative flow value is displayed, and the frequency current conversion circuit is converted into an analog current amount to display the instantaneous flow value.
Second, the application of turbine flowmeter
　　 1 , starting flow
The turbine blade works against friction and when the gas flow is small, the blade does not start to rotate, resulting in a metering dead zone in the turbine flow meter. When the gas parameters such as pressure, temperature, and density change, the initial flow rate of the turbine flow meter also changes accordingly. When the static pressure of the measured gas increases, the measured flow range also increases accordingly, and the density of the gas causes different pressure losses. Gas turbine flowmeters are suitable for applications where the flow rate is large, but when the gas volume is too small, the turbine flowmeter cannot overcome the frictional torque. After the gas volume is greater than the value, the flow rate has a linear relationship with the rotational speed; the air flow rate should not be too large due to the limitations of the impeller strength, the air flow pressure loss, and the bearing life. Most turbine meter at atmospheric pressure measuring range 1: 10 to 1:30, accuracy up to ± 0.5% ~ ± 1.0%.
　　 2 , installation requirements
Turbine flowmeters are very sensitive to the gas pipeline flow state, and the front end of the choke device the flow meter inlet (separation dust, pipe bends, cyclones, filters, etc.) may affect the flow state change of rotation of the rotor. For this reason, it is generally recommended to add a straight pipe section at the inlet and outlet ends of the turbine flowmeter, the length of the inlet end is not less than 10 times the inner diameter of the pipe section, and the length of the outlet end is not less than 5 times the inner diameter of the pipe section. If the installation space does not meet the above requirements, a rectifier can be installed between the flow blocking device and the turbine flow meter. In addition, the transmitter power cord needs to be metal shielded cable, and the grounding is good and reliable.
Third, failure analysis and maintenance processing
　　 The gas flows normally and the turbine flow meter does not count. Fault analysis: Whether the power line, function selection switch and signal line have broken or poor contact; whether the printed circuit board and the contact parts of the display are in poor contact; whether the coil is good; whether the impeller collides with the inner wall of the sensor, and there is no foreign matter stuck. The shaft and bearing are stuck or broken. Maintenance processing: use ohmmeter to check the fault point; replace the " replacement board " method on the printed circuit board fault , replace the fault board and make a detailed inspection; check whether the coil is broken or solder joints; remove foreign matter, clean or change Damage to the parts, after the recovery air blow or hand pull the impeller, there should be no friction sound, after the replacement of bearings and other parts should be re-verified to obtain a new meter factor.
Without the reduction operation, the flow display gradually decreased. Fault analysis: Whether the filter in front of the turbine flow meter is blocked; the valve opening on the flow sensor pipe section is automatically reduced; the sensor impeller is obstructed by debris or the bearing gap enters foreign matter, and the resistance increases to slow down the speed. Operation and maintenance: clean the filter; adjust the upstream valve of the flowmeter to determine whether there is any fault, and then repair or replace it after confirmation ; remove the sensor debris and re-calibrate.
　　 The gas does not flow and the flow display is not zero. Fault analysis: Poor shielding of transmission signal lines, external signal interference; external vibration of the pipeline, the impeller will be shaken, and false signals will be generated; internal leakage will occur when the shut-off valve is closed tightly; interference will occur between the display circuit boards or electronic components. Operation and maintenance: Check whether the power cable and signal cable are laid separately, whether the grounding of the display terminal is good; strengthen the pipeline, install equipment to prevent vibration; repair or change the shut-off valve; check the display circuit, judge the interference source, and find the fault point.
Fourth, matters needing attention
　　 (1) Before the turbine flowmeter is put into use, the meter factor is reset by the control panel of the flow computer.
　　 (2) When the turbine flowmeter is put into operation, the pressure difference between the front and the back should not be too large, and the gas flow rate should not be too fast. The inlet valve should be manually opened slowly. After the pipeline is full of gas and the pressure is balanced, the outlet valve is manually opened to prevent ice blockage. And the instantaneous airflow impact damages the turbine.
　　 (3) Regularly clean, inspect and re-calibrate the turbine flowmeter, and regularly inject lubricating oil to maintain the impeller's good operation; pay attention to the flow display status of the flow computer, evaluate the flow situation, and check immediately if there is an abnormality.
　　 (4) Keep the pipeline open. The filter can be judged to be blocked according to the differential pressure value of the differential pressure transmitter at the inlet and outlet of the filter. When the gas flowmeter turbine malfunctions, the pipeline should be cleaned, the turbine of the flowmeter is removed, and then cleaned with light high-grade gasoline and reinstalled.

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